Metabolic syndrome is a group of syndromes characterized by central obesity (high body weight), hypertension, hyperglycemia, hyperlipidemia and insulin resistance (hyperinsulinemia), commonly known as the “five highs”. The National Cholesterol Education Program (NCEP) recommends that those who meet the “three highs” of the “five highs” be classified as metabolic syndromes. These metabolic abnormalities are important risk factors for cardiovascular diseases, diabetes, gout, and steatohepatitis, so some experts have called these “death quartets” in urban populations, which shows the potential danger of metabolic syndrome. According to China’s metabolic syndrome diagnostic criteria, people with three or more of the following indicators are metabolic syndrome patients: overweight and/or obese, body mass index (BMI) ≥ 25 (convert weight into kilograms, height into meters, and then divide weight by height squared, that is, the body mass index); fasting blood glucose ≥ 6.1 mmol/L and/or two-hour postprandial blood glucose ≥ 7.8 mmol/L; systolic blood pressure ≥ 140 /l; systolic blood pressure ≥ 140 mm Hg, diastolic blood pressure ≥ 90 mm Hg; triglycerides ≥ 1.7 mmol/l and HDL cholesterol < 0.9 mmol/l for men or HDL cholesterol < 1.0 mmol/l for women. The 2002 nationwide survey on the nutrition and health status of Chinese residents based on this criterion showed that about 90% of patients with metabolic syndrome nationwide had elevated blood pressure, more than 80% had dyslipidemia, and about 45% had abnormal glucose metabolism. Research shows that overweight and obesity are the most important risk factors for metabolic syndrome, and the incidence rate of overweight people is 30 times higher than that of normal weight people, while the incidence rate of obese people is 76 times higher than that of normal weight people. As age increases, the relationship between overweight and obesity and the incidence of metabolic syndrome becomes stronger. Studies have also confirmed that people with metabolic syndrome are five to six times more likely to have a stroke than people without metabolic syndrome. Experts warned that the incidence of metabolic syndrome in people over 18 years of age in China has reached 6.6%, with more than 60 million people suffering from the disease. Therefore, the correct understanding and prevention of metabolic syndrome can not be delayed. Although the pathogenesis of metabolic syndrome and each of its components are complex, many studies have found that central obesity and insulin resistance are the basic factors in the development of metabolic syndrome, while the oxidative damage of oxygen free radicals is the "second blow", which plays a role in adding insult to injury. So what exactly is oxygen free radical and how is it intrinsically linked to the metabolic syndrome?